This invention relates to hydraulic fluid pressure energy translating devices. In particular, this invention relates to parallel disposed multi-piston pumps of the type used to convey hydraulic fluids, or the like.
Conventionally, such pumps comprise a rotatable cylinder block or barrel containing a plurality of parallel axially-extending pistons disposed equi-angularly about the central axis of the block. A cam plate is provided at one end of the barrel for engaging bearing shoes provided on each of the pistons and for respectively reciprocating each piston on rotation of the barrel to pump fluid into and out of each piston-containing cylinder. Such pumps are generally equipped with ported plate means for providing a valving action for ingress and egress of fluid into and out of the pump. The port plate further typically provides a bearing area against which the axial thrust of the barrel may be partially absorbed; this bearing area is lubricated with an oil film at the interface of these surfaces which minimizes wear of these elements and prevents seizure.
Although it is highly desirable from the standpoint of control of fluid leakage and the maintenance of adequate lubrication to have port plate and barrel mating surface properly aligned, this alignment is usually difficult to maintain during operation of the pump due to the well-known propensity of the barrel to tilt in a radial direction. This effect is primarily caused by the difference in longitudinal location of the radial components of force acting on the barrel through the pistons during rotation. When the barrel tilts as a result of these forces, the mating surface of the barrel and bearing surface of the port plate are urged out of alignment into an inclined position, resulting in undesirable fluid leakage, and disruption of the lubricating oil film.
Additionally, in conventional axial piston-type pumps, the barrel has a tendency to move axially against the bearing surface of the port plate during operation of the pump in response to axial components of force generated by the pressurized fluid in the cylinders. This net force urging the barrel against the port plate, generally termed the hydraulic clamp force, tends to reduce clearance between the bearing surface of the port plate and the barrel mating surface, and acts in opposition to the provision of an oil film at this interface thereby decreasing lubrication between the barrel and port plate. In the event, for example, of transient over pressure in the cylinders, the hydraulic clamp force may significantly increase, and the concomitant decrease in lubricating oil film at this interface results in wear and tear on these elements, and occasional seizure.
These radial and axial components of force acting upon the barrel are frequently particularly pronounced in pumps of larger size. Increased rotational speeds are required in order to provide the fluid necessary to effectively actuate the associated implements. As the rotational speeds increase, the forces tending to cause axial and radial displacement of the barrel also increase, disrupting the alignment and the clearance between the bearing surface of the port plate and the mating surface of the barrel, or "bearing clearance", thereby resulting in fluid leakage and/or insufficient lubrication at the port plate/barrel interface, as described above.
Although these problems have been recognized in the prior art, and numerous solutions proposed therefor, such as those described in U.S. Pat. Nos. 3,183,846; 2,735,407; 3,267,871 and 3,292,553, including means for laterally stabilizing the cylinder barrel such as described in U.S. Pat. No. 3,267,871 and floating valve means such as described in U.S. Pat. No. 2,649,741, none of these known prior art solutions have proved entirely satisfactory. For example, difficulties have been encountered in resolving the attendant problems of maintaining satisfactory bearing clearance under variable operating conditions in pumps having rigidly positioned barrels, wherein substantial variations in temperature, pressure, rotational speed, vibrations and other factors occur. It is highly desirable that means be provided for simply and effectively compensating for the forces developed within the pump to maintain adequate bearing clearance during pump operation, for controlling fluid leakage which impairs the efficiency of the pump, and for accomodating an adequate lubricating oil film between the port plate/barrel interface which prevents excessive wear, or in extreme cases, seizure of these elements.